In the industrial production of components, during surface machining damage may occur. Particularly in the mass production of components it is therefore necessary by means of a suitable inspection method to recognize such damage so that the damaged components can be separated out and the production process modified so as to avoid it.
A frequently occurring form of surface damage is the so-termed grinding burn, which for example can take place during tooth grinding to produce the final contour of toothed components. For the recognition of surface damage or grinding burn, from the prior art, chemical etching methods are known. During so-termed Nital etching the component to be inspected is treated with various acids and electrolytes such that after the treatment surface damage and particularly grinding burn can be recognized by virtue of color changes on the component. Nital etching is on the one hand not environmentally friendly, and on the other hand it incurs very high costs for the maintenance and disposal of the acids and electrolytes used. Furthermore, it is a subjective inspection method in which the surface damage is assessed by human experts by virtue of the discoloration. Thus, Nital etching is not suitable for inspecting the surface of components in the context of mass production.
From the prior art, the so-termed Barkhausen Noise Analysis is also known, with which the surface of ferromagnetic materials can be inspected by means of appropriate sensors. In the documents EP 0 100 009 A1 and DE 43 33 830 A1, methods for surface analysis taking account of Barkhausen noises are described. Barkhausen Noise Analysis is based on the so-termed Barkhausen jumps, which are produced by the reversal of magnetization in ferromagnetic materials in a magnetic field of slowly increasing strength. Ferromagnetic materials consist of small magnetic regions (called domains) with a uniform magnetization direction. The domains are separated from one another by Bloch walls, within which the magnetization direction changes through the width of the walls. A movement of the Bloch walls can be brought about by an external magnetic field. If at the same time a coil is positioned close to the ferromagnetic component, the magnetization produces an electric pulse in the coil. Addition of the impulses produces a noise-like signal, also known as Barkhausen noise or a Barkhausen noise signal. The Barkhausen noise signal depends on the surface condition and in particular on the surface hardness. By means of the Barkhausen noise signal grinding burn can be detected, since grinding burn reduces the surface hardness.
Since the Barkhausen noise depends not only on the surface condition of the component but also on other influencing parameters and particularly on the component geometry, a suitable calibration of a measuring device for Barkhausen noise analysis is necessary for the specific component geometry concerned. Here and in what follows ‘component geometry’ is understood to mean the specification of the dimensions of the component, as defined for example on a design drawing. Conventionally, calibration for a specific component geometry is a very elaborate process since for this several components with that geometry have to be used, which have to be made deliberately with different degrees of surface damage, for example different grinding burn severities. For this the components can for example be produced by grinding them with graded feed rates. The components so prepared are then inspected using the measuring device to be calibrated, i.e. the corresponding Barkhausen noise signals are recorded by the measuring device. Then, the components are graded in terms of their surface damage by Nital etching so that the surface damage can be correlated with the Barkhausen noise signals. As mentioned, this calibration process has to be carried out for each component geometry. Moreover, the calibration must be repeated whenever the sensor element of the measuring device is replaced. Furthermore, the controlled production of surface damage or grinding burn is very difficult in practice. Thus, Barkhausen noise analysis is not well suited for serial measurements of components having numerous variants.